A series of relatively rigid molecules, from the endoethenooripavine and endoethanooripavine families of opioids known to bind to opiate receptors from rat brain cerebrum homogenates and neuroblastoma-glioma hybrid cells, were converted to ligands potentially capable of irreversible interaction with those receptors. At least one of them has been found to act as an irreversible binding ligand for the delta opiate receptor and is being used to differentiate these receptors from other opiate receptors. Another endoethenooripavine has been found to bind to mu receptors and delta receptors. Its binding to delta receptors is irreversible.